I want to create a drop-down filter for Page - where the user can search for and select a page to filter a panel. I don't want to have to drag in individual pages (there are many hundreds), I just want a list of all pages that the user can search through. Alternatively, is there any way to 'filter' filters? i.e. can I apply a segment to a drop-down filter so it only shows pages that meet certain criteria?
Maybe you do not need the segment (drop-down) at all. Why not to click on the funnel icon next to the Page dimension in the freeform table and apply a filter? — yes, it will be manual process, but it's very universal.
Alternatively, train your users to use to left rail. Find the Page dimension, click on the arrow icon next to it, search for a page, drag and drop it into the freeform table.
For the "how to filter a filter" part of your question, you can multi-select specific values in the left rail (eg// specific pages), hold down the <shift> key and drag them over (as a group) to create a drop-down filter containing just those items. This isn't practical for a large number of items, but works well for a curated list.